1. Field of the Invention
This invention relates to an optical amplifier. In particular, it relates to an optical amplifier that has a function for monitoring an input light level.
2. Description of the Related Art
The amount of information sent and received via networks is increasing rapidly. In addition, as the information becomes more international in character, the importance of long-range communications is rapidly increasing. In this kind of long-range transmission, particularly when a large amount of information is transmitted, optical fiber cables are used. However, when signals are transmitted via optical fibers, as the transmission distance increases, the signal is attenuated. For this reason, in long-distance optical transmission, relay nodes containing optical amplifiers are normally installed at specified intervals. The signal light is amplified at each node and then sent to the next node.
A number of types of optical amplifier have been developed. One of the types is the optical fiber amplifier. In particular, in the 1550 nm band, optical amplifiers using rare earth-doped optical fiber, into which rare earth elements such as erbium have been doped, are widely used. In a rare earth-doped optical fiber, the rare earth elements are excited by excitation light which is input apart from the signal light, and the signal light passing through the optical fiber is amplified by the excitation energy.
Some optical amplifiers have a mechanism to monitor the input light level. That is to say, the input light level is monitored and, whether or not its level drops below a threshold level is monitored. Conceivable reasons why the input light level might drop below the threshold include: (1) Light is not being output from the sending side, or is not arriving for some reason, such as the optical fibers are broken, (2) Although light is being output from the sending side, that light does not include a signal that contains information to be transmitted.
Thus, since, if light or a signal is not being transmitted, it is not necessary for the optical amplifier to amplify anything, the excitation light that excites the rare earth elements is stopped. This unit composition conserves the power used to drive the light source (normally a laser) that outputs the excitation light. In addition, since the amplification action inevitably generates noise, stopping the amplification action prevents the optical amplifier itself from becoming a noise source.
In an optical communication system that uses optical amplifiers, the noise that is generated in the different amplifiers accumulates. For this reason, in particular in a transmission area where many optical amplifiers are connected in series, it is necessary to suppress the amount of noise generated in each optical amplifier as much as possible. The amount of noise generated in an optical amplifier is expressed as the S/N (signal-to-noise ratio) of the output light relative to the S/N of the input signal, and is called the noise index.
In an optical amplifier having the above composition, the mechanism that monitors the input light level is one of the causes that prevents the noise level from being reduced. That is to say, in order to monitor the input light level, normally an optical splitter, for example, is used to branch off part of the input light; then the level of this branched-off part of the input light is detected by, for example, a photodiode, and the input light level is computed. For this reason, part of the input light is lost without being transmitted to the output side, causing the noise index to become worse. In particular, for example, if the transmission path on the input side is long, when the input light level is low, in order to measure that light level accurately, the amount of light that is branched off to the photodiode side must be kept at or above a certain fixed level, thereby decreasing the amount of light that is actually amplified for transmission and making the noise index even worse.
In order to deal with this problem, a configuration in which the input light level would be monitored indirectly has been proposed. That is to say, when the input light includes a signal, the input light is amplified while the excitation light power used to excite the rare earth elements is held fixed; the output light is branched off, and the level of the branched light is measured to compute the input light level. However, in this configuration, if the excitation light is stopped to conserve power when the input light does not include a signal, the input light is passing through an optical fiber which contains unexcited rare earth elements, in which case the attenuation (loss) will be large. For this reason, when the input light switches from not containing a signal to containing a signal, there is a danger that the monitoring mechanism on the output side of the optical amplifier will not be able to detect that change of condition. In this case, the excitation light is not output and the input light cannot be amplified. Consequently, in a configuration in which the input level is indirectly monitored by measuring the level of light output from the optical amplifier, it is necessary to constantly supply excitation light to the rare earth-doped optical fibers, preventing power from being conserved.